It is known to provide an apparatus for monitoring torsional loading of a drive shaft by utilizing magnetostrictive effects induced therein includes a stationary ring surrounding the shaft and carrying a multiplicity of inwardly radially oriented pole pieces having respective energizing coils, the coils being fed, for example, from the electrical network of an automobile. A pair of secondary rings flank the primary ring and bear coil-wound pole pieces whose inner ends are juxtaposed to the outer surface of the shaft. The coils on the secondary rings are wound in opposing directions and are included in the same electrical circuit.
When the shaft is unloaded, the magnetic field generated by the primary or magnetizing poles is symmetric and lies exactly between the secondary or sensing poles. In this case the magnetic flux through the secondary coils and the current in the sensing circuit are zero.
A torsional loading of the drive shaft results in differential changes in the magnetic permeability. Along the directions of tensile stressing, the magnetic permeability increases, while in the directions of compression, the permeability drops. This alteration of the magnetic permeability of the shaft tilts the direction of the field lines generated by the primary coils, thereby producing a magnetic flux in the secondary pole pieces and through their respective sensing coils. The resulting current or voltage signal in the secondary circuit is a function of the torsional stressing of the drive shaft.
A disadvantage of such an electromagnetic torsion monitor is the large amount of space and energy required in order to achieve measurable magnetic flux densities within linear hysteresis ranges. One reason for the exorbitant energy requirement arises from the heat treatment of the shaft. In addition to increasing the strength of the shaft, heat treatment facilitates the penetration of the shaft by the magnetic field lines and thereby reduces the field density in the region of the sensing coils.
The space and energy requirements render the torsion monitor impractical, uneconomical and unusable in many applications.